The Influence of Tungsten Alloying Additions on the Passivity of Aluminum

Transition metals such as Mo, W, Ta, Cr, and W can be used to significantly enhance the localized corrosion resistance of Al. Since these elements exhibit very low solubilities in aluminum, a nonequilibrium alloying method, such as sputter deposition, must be used to produce single-phase alloys. The addition of approximately 9 atom percent W to Al can shift its pitting potential in the positive direction as much as 2600 mV, Heat-treatment of these nonequilibrium alloys results in the precipitation of a second phase that is detrimental, but not catastrophic, to corrosion performance. Surface analysis of the Al-W passive film formed as a function of applied anodic potential reveals that it remains thin throughout the polarization sequence and contains very little oxidized solute, regardless of the applied potential. The oxidized W present in the film is in the form of WO² and WO³ (or WO₄^-2). Previously, it was proposed that the enhanced passivity for sputter-deposited Al-Mo, Al-Cr, and Al-Ta alloys was a result of the oxidized solute in the film (M00₄^-2, CrOOH, or Ta²O⁵), making the film less susceptible to chloride attack by electrostatic-repulsion, oxide-structure modification, or oxidized-solute barrier-layer formation mechanisms. The results of this investigation reveal that the electrostatic-repulsion or oxidized-solute barrier-layer mechanisms are not responsible for the dramatic enhancement in the passivity of Al with the addition of W. Instead, the small amount of oxidized W in the passive film may interact synergistically with the hydrated aluminum oxide structure to form a more protective film.